Extrusion molding of inorganic masses, such as ceramic-forming materials has been performed by passing a green body or composition, which is obtained by mixing and kneading adjuvants such as organic binders, surfactants, lubricants, and plasticizers with inorganic materials, particularly ceramic-forming materials, through dies having a desired shape into a sheet, a bar, a hollow tube, a rectangular column, a hollow rectangular column, or a honeycomb structure. In particular, the extrusion-molded body in the form of ceramic honeycombs has been in use as a carrier for exhaust gas cleaning catalysts, filters, and heat exchangers in the fields of automobiles and various industries.
A well-known adjuvant is methyl cellulose. It forms strong gels at elevated temperatures. Methylcellulose in the extrusion molding of inorganic masses is desired for improving the wet green modulus and wet green strength of the extruded mass, often colloquially referred to as “green strength” Improved green strength facilitates the extrusion of thin-walled honeycomb structures from ceramic batches and reduces production failures. Unfortunately, the well-known low gelation temperature of methyl cellulose has disadvantages in some processes for extrusion-molded bodies.
U.S. Pat. No. 4,551,295 relates to the extrusion of a plastic ceramic batch into articles of widely-differing profiles and shapes such as, for example, dinnerware and electrical insulators, and especially the extrusion of thin-walled honeycomb structures. The U.S. patent discusses that a methyl cellulose, such as METHOCEL™ A4M cellulose ether having a viscosity of 4000 mPa·s, measured as a 2 wt. % aqueous solution at 20° C. according to Ubbelohde, has a low gelation temperature. According to FIG. 8 of the U.S. patent a sharp rise in extrusion pressure is observed when increasing the extrusion temperature in the range of 23-30° C. The U.S. patent discusses that such rise in extrusion pressure is not observed when using as a binder/plasticizer METHOCEL™ F4M cellulose ether which is commercially available from The Dow Chemical Company and has a viscosity of 4000 mPa·s, measured as a 2 wt. % aqueous solution at 20° C. according to Ubbelohde. METHOCEL™ F4M cellulose ether has a methoxyl substitution of 27.0-30.0 weight percent and a hydroxypropoxyl substitution of 4.0-7.5 weight percent. The U.S. patent suggests using a hydroxypropyl cellulose having a viscosity of 25,000-100,000 mPa·s, measured as a 2 wt. % aqueous solution at 20° C. according to Ubbelohde to permit the use of working temperatures greater than 35° C. in a twin screw extrusion apparatus.
Unfortunately, hydroxyalkyl methylcelluloses are known to have a low storage modulus, compared to methyl cellulose. Hydroxyalkyl methylcelluloses which exhibit a low storage modulus do not form strong gels. High concentrations are needed to form even weak gels (Hague, A; Richardson, R. K.; Morris, E. R., Gidley, M. J and Caswell, D. C in Carbohydrate Polymers 22 (1993) p. 175; and Hague, A. and Morris, E. R. in Carbohydrate Polymers 22 (1993) p. 161). For example, at the same concentration of 2 wt.-%, at elevated temperatures the maximum storage modulus of a METHOCEL™ K4M HPMC is typically less than about 100 Pa, whereas that of a METHOCEL™ A4M methylcellulose is typically above about 1000 Pa. It is concluded that the hydroxyalkyl substituents inhibit intermolecular associations.
In line with the above-mentioned teachings, gel strength is known to be lost with rising hydroxyalkyl substitution of known hydroxyalkyl methyl celluloses. N. Sarkar discusses in the Journal of Applied Polymer Science, 24 (1979), pp. 1073-1087 the thermal gelation properties of methylcellulose and hydroxypropyl methylcellulose. FIG. 9 of the article, which is included as FIG. 1 in the present patent application, illustrates the gel strength of 2 weight-% aqueous hydroxypropyl methylcellulose (HPMC) gels after 4 hours at 65° C. as a function of hydroxypropyl molar substitution. The graph illustrates that gel strength of a HPMC with an MS (hydroxypropyl) of 0.15 is only about one third of the gel strength of methylcellulose (MC).
To resolve the issue of low gel strength of hydroxyalkyl methyl celluloses, European Patent Application EP 1 983 004 discloses a water-soluble hydroxyalkyl methyl cellulose having a molar substitution of hydroxyalkyl groups of 0.05 to 0.1 and a substitution degree of methoxyl groups of 1.6 to 1.9, wherein the hydroxyalkoxyl groups are classified into substituted hydroxyalkoxyl groups having hydroxyl groups of hydroxyalkoxyl groups substituted further with methoxyl groups and unsubstituted hydroxyalkoxyl groups having hydroxyl groups of hydroxyalkoxyl groups not further substituted; and a ratio (A/B) of a molar fraction (A) of the substituted hydroxyalkoxyl groups to a molar fraction (B) of the unsubstituted hydroxyalkoxyl groups is 0.4 or greater. However, it would be desirable to increase the gel strength in another way than only lowering the molar substitution of hydroxyalkyl groups because lowering the molar substitution of hydroxyalkyl groups in hydroxyalkyl methyl celluloses typically leads to an increased fiber content, which is undesirable in some applications where hydroxyalkyl methyl celluloses are used, such as extrusion-molded bodies having a ceramic honeycomb structure.
To optimize the use of cellulose ethers as adjuvants in the extrusion of inorganic masses, such as ceramic-forming materials, it would be desirable to provide new cellulose ethers, particularly new hydroxyalkyl methylcelluloses, which have a higher gel strength than known comparable hydroxyalkyl methylcelluloses.
Moreover, it would be desirable to provide new cellulose ethers which are useful as an adjuvant in inorganic masses to produce extrusion-molded bodies and which provide the produced extrusion-molded bodies with a higher wet green modulus than when known hydroxypropyl methylcelluloses are included in the inorganic mass.